1. Field of the Invention
The present invention relates to a method for fabricating a liquid crystal display device, and more particularly to a method for forming a polycrystalline silicon film in order to fabricate a polycrystalline silicon thin film transistor.
2. Description of the Prior Art
A thin film transistor (hereinafter, simply referred to as “TFT”) used as a switching device in a liquid crystal display device or an organic light emitting display is a main component for improving performance of such flat panel display devices. Herein, mobility or current leakage, which is a basic factor determining performance of the TFT, varies depending on a state of an active layer providing a route for a charge carrier. That is, such mobility or current leakage may vary depending on a state or a structure of a silicon thin film forming the active layer. In currently available liquid crystal display devices, the active layer of the TFT is made from amorphous silicon (hereinafter, simply referred to as a-Si)
However, an a-Si TFT including a-Si as an active layer has low mobility of about 0.5 cm2/Vs, so problems may occur if all switching devices of the liquid crystal display device are fabricated using an a-Si TFT. That is, a driving device for a peripheral circuit of the liquid crystal display device must be operated with a high speed, but the a-Si TFT cannot satisfy the operational speed required by the peripheral circuit of the liquid crystal display, so a problem may occur if the driving device for the peripheral circuit is fabricated by using the a-Si TFT.
Meanwhile, a poly-Si TFT including polycrystalline silicon (hereinafter, simply referred to as poly-Si) as an active layer has high mobility of about tens of or hundreds of cm2/Vs. Thus, the poly-Si TFT can satisfy the high operational speed required by the peripheral circuit of the liquid crystal display. Therefore, it is possible to achieve a pixel switching device as well as driving parts for the peripheral circuit by forming the poly-Si TFT on a glass substrate. Accordingly, a module process for the peripheral circuit is not required and costs for the driving parts of the peripheral circuit can be saved because the driving parts of the peripheral circuit can be simultaneously formed when forming a pixel region.
In addition, the poly-Si TFT can be fabricated with a small size as compared with the a-Si TFT due to high mobility of the poly-Si TFT. Furthermore, since the driving device of the peripheral circuit and the switching device of the pixel region can be simultaneously formed through an integration process, a micro design rule can be easily achieved so that an a-Si TFT-LCD can provide an image having high resolution.
Moreover, since the poly-Si TFT has a superior current characteristic, the poly-Si TFT is adaptable for a driving device of an organic light emitting display, which is a next-generation flat panel display device. Thus, studies and research regarding the poly-Si TFTs have been actively carried out to fabricate the poly-Si TFTs by forming a poly-Si film on a glass substrate.
In order to form such a poly-Si film on the glass substrate, an a-Si film is deposited on the glass substrate and a heat-treatment process is carried out with respect to the a-Si film, thereby crystallizing the a-Si film. However, in this case, the glass substrate may be deformed if a process temperature exceeds 600° C., thereby lowering reliability and productivity.
Thus, an excimer laser annealing process has been suggested in order to crystallize the a-Si film without causing a terminal damage to the glass substrate. In addition, a sequential lateral solidification (hereinafter, simply referred to as “SLS”) method has been suggested.
According to the SLS method, poly-Si is formed by crystallizing a-Si using a pulse layer and a mask having a slit pattern providing a transmission route for a pulse laser beam. In this case, a crystallization status of poly-Si may vary depending on a shape of the mask and a proceeding route of the laser beam.
According to the SLS method, a seed poly-Si film is firstly formed and a next poly-Si film is grown based on the seed poly-Si film through a directional process, a 2-shot process, a 3-shot process, and an n-shot process.
In a first shot of the 3-shot SLS method, poly-Si grains growing from a slit pattern of a first shot, that is, poly-Si grains laterally growing from both sides of a transmission section make contact with each other at a center portion of the transmission section while forming a protrusion so that the growth of the poly-Si grains may stop at the center portion of the transmission section. In addition, in a second shot of the 3-shot SLS method, poly-Si grains are grown based on seed poly-Si grains, which have been grown in the first shot, and the growth of the poly-Si grains may stop when poly-Si grains laterally growing from both sides of the transmission section make contact with each other. In addition, in a third shot of the 3-shot SLS method, poly-Si grains are grown based on seed poly-Si grains, which have been grown in the first and second shots, and the growth of the poly-Si grains may stop when poly-Si grains make contact with each other.
However, according to the 3-shot SLS method, the growth of the poly-Si grains laterally growing from both sides of the transmission section may stop when the poly-Si grains make contact with each, so the poly-Si grains cannot be grown in a maximum size. Accordingly, a size of the poly-Si grains may be limited, so performance of a poly-Si TFT is also limited.